Method for producing microdot-emitting cathodes on silicon for compact flat screens and resulting products

ABSTRACT

A method for producing microdot emitting cathodes on silicon for compact flat screens, and the products obtained by means of said method, are disclosed. According to the method, the emitting cathodes are made from a basic monolithic silicon substrate (1) consisting of a thick wafer (at least 300 microns) or a thin film a few microns thick on an insulating substrate (alumina or glass), the silicon film being &#34;active&#34; in both cases. The method is useful in the field of flat display screens based on the physical phenomenon of cathodoluminescence and field effect electron emission, and in all industrial sectors using compact display screens, e.g. video camera viewfinders, calculators, monitoring devices of all kinds, vehicles, watches and clocks, etc.

FIELD OF THE INVENTION

The present invention relates generally to microtip-emitting cathodes onsilicon for compact flat screens.

More specifically, the present invention relates to flat display screensbased on the physical phenomenon of cathodoluminescence and field effectelectron emission. Further, the present invention can be applied in allindustrial sectors using compact display screens, for example, videocamera view finders, calculators, monitoring devices of all kinds,vehicles, watches, and clocks, etc.

BACKGROUND OF THE INVENTION

The microtip screens are characterized by an electronic field effectemission from an extended plane microtip cathode, a low consumption coldcathode, a rapid response time (1 μs), a matrix addressing from theintegrated tip-grid structure and a luminous emission bycathodoluminescence at a low/average voltage.

Known microtip screens are vacuum tubes generally constituted of twothin glass plates (approximately 1 mm), distanced by 200 μm. Therigidity of the structure is ensured by spacers (balls of 200 μm, forexample) which enable the interelectrode distance to be maintained whenthe screen is placed under vacuum.

The front plate or anode plate is covered by a transparent conductinglayer and luminophores.

The rear plate or cathode plate comprises a matrix network of fieldeffect emitters deposited by thin film technology.

Each luminous dot (pixel) is associated with an oppositely locatedcathodic emitting surface and constituted of a large number of microtips(approximately 10,000 per mm²).

This emitting surface is defined by the intersection of a line (grid)and a column (cathodic conductor) of the matrix.

Subject to the introduction of a device for limiting the current in thetips, the large number of tips ensures a homogeneous emission betweenpixels (average effect) and eliminates the risks of local defects.

By virtue of the short tip-grid distance (≦1 μm) and the amplifyingeffect of the tip, a potential difference of less than 100 volts appliedbetween line and column enables obtention, at the top of the tip, of anelectric field greater than 10 to the power of 7 volts/cm, sufficient tocause the emission of electrons.

To fix the order of magnitude, a potential difference of 80 volts allowsa current density of 1 mA/mm² to be obtained. This value is sufficientin a screen of 1,000 lines, controlled sequentially line by line toobtain a high luminance (400 cd/m²) with a low voltage luminophore (400volts) having a luminous yield of 3 lm/watt.

In light of the emission threshold (40-50 volts), the voltage which mustbe modulated on the columns to pass from the black level to the whitelevel, is of the order of 30 to 40 volts.

The conventional structure of the cathode of a microtip screenespecially comprises, deposited successively on a substrate of glass orsilicon:

an insulation layer,

a resistive layer of silicon or other material,

"column conductors" constituted of a metallic layer which can bedeposited either beneath or above the resistive layer,

an insulating layer (Si or SiO2) which constitutes the grid insulator,

a metallic layer which constitutes the grid.

After depositing the aforementioned layers, holes on which the microtipsare then produced, are drilled into the grid and the grid insulators byknown etching techniques.

SUMMARY OF THE INVENTION

The method according to the present invention leads to an improvement ofthe characteristics, as well as better manufacturing yields in theproduction of microtip-emitting cathodes for compact flat screens of thecathodoluminescence type, and allows the use of known techniques forforming components in silicon.

It consists of producing emitting cathodes from a basic monolithicsilicon substrate consisting either of a thick wafer (300 microns ormore) or a thin film a few microns thick, deposited on an insulatingsubstrate (alumina or glass), the silicon film being "active" in bothcases.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed schematic drawings, provided as a non-limiting example ofone of the embodiments of the object of the invention:

FIG. 1 represents the transverse section of a microtip-emitting cathodeaccording to the invention,

and FIG. 2 is a top view of such a cathode showing a special embodimentof the column conductors.

DETAILED DESCRIPTION OF THE DRAWINGS

The method according to the present invention is intended to producemicrotip-emitting cathodes for compact flat screens using a basicsilicon substrate 1 consisting either of a thick wafer (300 microns ormore), or a thin film a few microns thick, deposited on an insulatingsubstrate (alumina or glass). In both cases, the silicon layer can beused advantageously to implant active components, such as depletiontransistors ensuring control and limitation of the current in themicrotips.

The emitting cathodes can be manufactured by known techniques forproducing integrated components on silicon. In addition, thecollectivization of treatments allows several cathodes to bemanufactured at the same time on the same wafer, and several wafers tobe treated at the same time during technological stages.

The thick wafer is constituted of a massive silicon plate having adiameter of 100 to 200 mm (but non-limiting), of the type commonly usedfor manufacturing integrated circuits. It is of the P- or N-type with anadapted, preferably high, resistivity. It can also be made of aninsulating substrate (glass, alumina, etc. . . . ) covered by a layer ofsilicon approximately 1 micron thick, or else by any kind of knownsubstrate allowing silicon structures to be produced on an insulator.

As for the thin film of silicon, the basic substrate can be a plate ofsilicon, alumina, glass, or other. The thin film itself is crystalline(epitaxial layer) or polycrystalline, having a high resistivity (from afew ohms-cm to 50 ohms-cm).

At each manufacturing stage, the cleaning phases are identical to thosewhich precede the stages of the integrated circuit production method.They consist of immersion in acid baths (phosphoric, hydrochloric,hydrofluoric, sulfuric) rinsing with deionized water, drying becentrifuge or alcohol vapor, etc. . . . .

FIG. 1 shows a partial section of an emitting cathode with microtipsprotected by depletion transistors, the latter being produced from thesilicon substrate 1 in which are formed over-doped zones, obtained bydiffusion and constituting sources 3 in contact with column conductors4, and drains 5 supplying microtips 2, as well as a grid insulationlayer 6 made of silica, obtained by surface oxidizing. A gate electrode7 is created by metallization above a gate insulation layer 6.

Column conductors 4 are constituted either by a metallic layer(aluminum, for example), or by one or more zones diffused in the siliconsubstrate, or by combining the two techniques: diffused layer+metalliclayer.

The use of a diffused layer allows the height of the structure to belimited.

The diffused layer can extend on the entire surface of column 9, toreduce its resistance. In that case, it is insulated from the upperstructures by a thick oxide layer (1 to 2 microns) in which contactholes 10 with the upper layers are formed. The diffused layer can alsobe limited at the surface of a pixel 11, column 9 then being constitutedof over-doped zones in series with metallic zones 12, which interconnectthe over-doped zones (FIG. 2).

If column conductor 4 is a metallic layer, one can use a structure whichseparates the first emitting tip from the column metallization by arequired distance (3 microns for example).

If the conductor column is a layer diffused in the silicon substrate,the same principle can be used to produce the same effect.

Both of the aforementioned principles (use of a diffused layer for thecolumn conductor and its alignment) enable the release of a maximumemitting space. In deed, in both cases, the encroachment of columnconductor 4 on the surface of the pixel is reduced to making contact.The conductor being either beneath the emitting zone (diffused layer) orin the inter-pixel space (metal).

Grid 8 (metallic) forming the line conductors, can be coveredadvantageously by an insulating layer (silicon nitride, diamond carbon,SiO2 or other). The insulation between grid 8 and anode is therebyimproved. This layer will usually be deposited before forming the holesand the microtips.

The positioning of various constituent elements provides the object ofthe invention with a maximum of useful effects which, to date, have notbeen obtained by similar methods.

I claim:
 1. A microtip emitting cathode for a flat display screen, comprising:at least one MOS transistor having a drain region coupled to a gate region; a cathode conductor; and an emitting microtip coupling said at least one MOS transistor with said cathode conductor.
 2. The microtip emitting cathode according to claim 1, wherein said at least one MOS transistor is formed on a silicon substrate, and said emitting microtip is disposed over said drain of said at least one MOS transistor.
 3. The microtip emitting cathode according to claim 2, further comprising a layer of metallization which couples said drain with said gate of said at least one MOS transistor, said microtip being disposed on said layer of metallization.
 4. The microtip emitting cathode according to claim 1, wherein said cathode conductor is column-shaped and comprises a plurality of sources which are coupled together.
 5. The microtip emitting cathode according to claim 1, wherein said at least one MOS transistor comprises a plurality of MOS transistors.
 6. A method for producing a microtip emitting cathode for flat display screen, comprising the steps of:providing substrate having at least one MOS transistor having a drain region coupled to a gate region; depositing a cathode conductor on said substrate; and coupling said at least one MOS transistor to said cathode conductor with an emitting microtip.
 7. The method according to claim 6, wherein said emitting microtip is disposed over said drain of said at least one MOS transistor.
 8. The method according to claim 7, further comprising the step of depositing a layer of metallization which couples said drain with said gate of said at least one MOS transistor wherein said microtip is disposed on said layer of metallization.
 9. The method according to claim 6, wherein said cathode conductor is column-shaped and comprises a plurality of sources which are coupled together.
 10. The method according to claim 6, wherein said at least one MOS transistor comprises a plurality of MOS transistors. 